I only have one character that has issues with exhaustion when fighting, 7th level, 12 con, BAB 4. The only reason she gets tired, is that fighting mobs of shadow fiends and greater shadow fiends when you only get one attack a round takes a long time. The times she did the sullen woods with a mage that could haste her, the fights lasted half as long, she only got half as tired, and could last from rest to rest before exhaustion set it.
I've been on four high/max spawn demonologist runs with Maria the last couple of weeks, and haven't heard any OOC comments about the exhaustion system during them, but those have been generally will decently balanced parties, resting as a group whenever it was needed.